ii PHYLUM PROTOZOA 99 



(Fig. 75), a parasite in the intestine of the Frog. Binary fission 

 takes place (D, E, F), and is repeated again and again so rapidly 

 that the daughter-cells are unable to grow to the adult size before 

 the next division. The final results of the process are small bodies 

 (G), each with only two or three nuclei instead of the large number 

 characteristic of the adult. These become encysted (H), and in 

 this passive condition are passed out of the Frog's intestine with 

 its faeces, frequently being deposited on water-weeds. All this 

 takes place during the Frog's breeding season : the tadpoles or Frog- 

 larvae feed upon the water-plants, and in doing so frequently take 

 in the spores or encysted Opalinoe along with their food. When 

 this occurs the cyst is dissolved by the digestive juices of the host, 

 and the protoplasm of the spore is set free as a rounded body 

 with a single nucleus (I), which rapidly grows into an adult 

 Opalina (K). 



Conjugation, in the form of a temporary union accompanied by 

 interchange of micronuclei, has been described in Paramoecium 

 (p. 90), and takes place in many Ciliata. In others (e.g. Stylonychia 

 histrio) there is a complete union of the two gametes. In 

 Vorticella union is also permanent, and takes place, not between 

 two ordinary forms, but between one of the ordinary stalked 

 individuals, or megagametes, and a free-swimming, small form, or 

 microgamete, produced, as described above, by budding (G 1 , G' 2 ). 

 The essence of conjugation is the reception of nuclear material 

 derived from another individual : its effect appears to be a renewal 

 of vitality, usually manifesting itself in increased activity in 

 multiplication by fission. 



ORDER 2. TENTACULIFERA. 



Judged from the adult structure alone, the members of this 

 order would certainly be placed in a separate class of the Protozoa : 

 it is only in virtue of the facts of development that they are 

 united in a single class with the Ciliata. 



The body may be globular (Fig. 76, la), ovoid (/b), or cup- 

 shaped (2a), but presents nothing like the variety of form met 

 with among the Ciliata. The distinguishing feature of the group 

 is furnished by the tentacles which are always present in greater 

 or less number, and which, in some cases at least, are the most 

 highly differentiated organs found in the whole group of Protozoa. 

 The characters of the tentacles vary strikingly in the different 

 genera. 



In the common forms Acineta (2), and Podophrya (1\ the ten- 

 tacles spring either from the whole surface, or in groups from the 

 angles of the somewhat triangular body. Each tentacle is an elon- 

 gated cylindrical structure (7c), capable of protrusion and retrac- 

 tion, and having its distal end expanded into a sucker. It is, more- 

 over, practically tubular, the axial region consisting of a semi-fluid 



H 2 



